Description of the Prior Art
Static electricity, more formally known as triboelectric charge, builds up naturally in many materials, for example as a result of simple friction between two dissimilar materials. The build up of static electricity is strongest in electrically insulative materials, as such materials hinder the flow and dissipation of the charge. Accumulated electrostatic charge discharges occur when a conductive path is established to an electrical ground. The conductive path may be nothing more than the surrounding air provided that the relative humidity is sufficiently high, and this is the reason why static charge seldom builds up in humid climates. On the other hand, very dry air acts as an insulator and inhibits the natural discharge of static electricity.
Modern electronic circuits make use of certain types of semiconductor devices, including integrated circuits and transistors, which can be irreparably damaged by even the briefest exposure to a minute electrostatic field or electrostatic discharge. Electrostatic damage to an electronic device is not visible or otherwise apparent, and typically is not discovered until the device is placed into operation, usually as part of a circuit board or other higher assembly. Consequently, unless the devices are individually tested, damaged devices or even entire circuit boards may not be discovered until they have been assembled into a system; therefore rendering the device inoperative and resulting in costly wasted time, labor and materials.
Because of this susceptibility, extreme caution must be taken at all stages of handling of such devices. Electrostatically sensitive devices must be protected at all times and at every step along the chain of industrial processing and commercial distribution up to their final installation in an end user system. This includes in-plant movement of the devices between different workstations as well as transport of the devices from an initial manufacturing site to a geographically removed assembly facility or some other commercial distribution point.
Considerable effort has been directed to the development of packaging materials capable of protecting the delicate electronic devices against damage by electrostatic discharge. This effort has been primarily directed towards improving the electrical conductivity of packaging material, specifically, the manufacture of electrically conductive papers and paperboards made from cellulosic pulp. The conductive sheet material is cut into single-piece blanks which are then folded along predetermined fold lines to make boxes, envelopes or other packaging. The one-piece construction of the packaging results in a continuous enclosure of electrically conductive material around the static-sensitive electronic devices.
Two primary approaches to the making of conductive packaging have been developed: impregnation of the paper material with a conductive substance, and coating or printing of the paper material with an electrically conductive ink. Depending on the degree of electrical conductivity of the resulting sheet materials they are classified, insofar as their anti-static properties, as either electrically conductive or electrostatically dissipative. Conductive materials are generally those having an electrical surface resistivity equal to or less than 10.sup.4 Ohms (ten thousand Ohms) per square, while dissipative materials have an electrical surface resistivity in the range of 10.sup.6 Ohms and 10.sup.12 Ohms per square (ASTMD-257-78). Conductive materials readily discharge static electricity and consequently prevent the build up of electrostatic charges. On the other hand, because of their superior conductivity, these materials can discharge accumulated static electricity so rapidly as to induce sparking, as for example, when a container of conductive material is touched in dry weather and a small spark jumps between hand and container. Sparking is potentially harmful to sensitive electronic devices even when the spark never touches the electronic device. The spark creates an electric field which in turn can induce damaging electrical charges in nearby electronic devices. It is therefore necessary not only to keep the electronic devices away from contact with any materials carrying an electro-static charge, but also to avoid exposure of the devices to the electric field of a nearby spark discharge.
Protection against electrostatic charge is normally provided by enclosing sensitive electronic devices and circuits in an electrically conductive enclosure. This arrangement is known in physics as a Faraday cage, which acts as an effective barrier against electrostatic charges. Static electricity may build up on the conductive enclosure, but does not penetrate the interior of the enclosure. However, the electronic devices are vulnerable to damage if sparking occurs between the container and a ground path while the container is open. Under such circumstances the open container does not completely shield the devices against the effects of electrical fields induced by the spark. While sparking from a closed conductive container to a ground path such as a person's hand cannot harm a sensitive device contained in the closed package, the electronic device can be damaged or destroyed if the same sparking occurs while the container happens to be open. The sparking problem can be avoided by use of electrostatically dissipative packaging materials which, because of their greater electrical resistivity, have a slower rate of discharge of the static build up. However, the Faraday shielding effect of dissipative materials is inferior to that of conductive materials.
Because of the above, considerable effort has been expended towards developing low-cost packaging materials, specifically papers and paperboards, which can be shaped into various types of boxes, envelopes and other containers, yet provide the protective Faraday cage effect to shield the contents against electrostatic charges. It is particularly desirable to make such conductive paper materials which are also recyclable, as very often such packaging is seldom reused. For example, U.S. Pat. Nos. 4,711,702 to Hood and 5,205,406 to Bradford describe processes for impregnating paper pulp with electrically conductive carbon black for making antistatic paperboard used in box containers and cartons for protecting electrostatic discharge sensitive devices. U.S. Pat. Nos. 4,160,503; 4,293,070 and 4,211,324, all issued to Ohlbach describe paperboard containers coated on inside surfaces with conductive carbon black to prevent an outside static electricity charge from reaching the sensitive contents. These efforts have produced packaging materials which are primarily electrically conductive in nature, and therefore offer less than complete protection of the contents because the possibility of sparking has not been eliminated.
Limited efforts have also been made to combine the advantages of each type of packaging material so as to provide full Faraday shielding while at the same time minimizing the risk of sparking. U.S. Pat. No. 4,000,790 issued to Youngs et al. describes a multi-ply fiberboard structure which includes one or more plies impregnated with a conductive material such as carbon black. The fiberboard is a sandwich structure made of multiple plies or layers which are pressed together to make a single thickness of the fiberboard. Conductive plies and unimpregnated plies may be assembled in various combinations. For example, a conductive ply may be placed between unimpregnated plies, which may be electrically dissipative due to the natural properties of the paper material. The result is a conductive layer buried between dissipative layers. Sheets of this multiply board can then be assembled to make corrugated board. For example, the multi-ply paperboard may be used as the inner and/or outer facing of a corrugated paperboard, in which the fluted medium or inner layer is typically made of kraft paperboard medium.
This arrangement suffers from the shortcoming common to containers made of fiberboard made conductive by impregnation with carbon black, namely, a susceptibility to sparking when the carbon material is closely adjacent to either the inner or outer surface of the paperboard. In a multi-ply board where the conductive ply is buried between non-conductive plies, conductive carbon material may be exposed at fold lines where the outer non-conductive plies may break open. Furthermore, such materials are susceptible to shedding of conductive carbon particles, either by migration through the nonconductive plies or at open fold lines. Such conductive particles can bridge closely adjacent conductors on electronic devices or circuit boards in the packaging and cause a destructive short circuit when the device is placed into service. Such particles are typically too small to see before irreparable damage has been done to the electronic device. This is referred to as latent defect.
What is needed is a paper or fiberboard material, particularly a corrugated fiberboard having adequate strength and rigidity for use in making packaging such as cartons, boxes, envelopes and other containers for electrostatically sensitive electronic devices, which provides Faraday cage type shielding against electrical fields and static discharges, which is safe against sparking, and which is also secure against shedding of conductive particles onto the contents of the package. It is further desirable that the improved packaging be available in large commercial quantity with uniform electrostatic properties and protective between 10.sup.7 Ohms and 10.sup.11 Ohms/square at a relative humidity greater than 10 percent. In a presently preferred form of the invention, the electrical resistivity of the kraft liner is between 10.sup.7 Ohms and 10.sup.10 Ohms at a relative humidity greater than 10 percent. Furthermore, it is desirable that the kraft liner have a static charge drain rate from 5 Kilovolt to 500 Volts in less than 2 seconds.
In a preferred form of the invention, the kraft liners each have an outer surface facing away from the corrugated medium, which outer surface may be coated with an electrically dissipative sealer. The dissipative sealer may include a dissipative ink and a sealant varnish such as a styrene acrylic polymer varnish. The dissipative ink may be applied in admixture with the sealer varnish to the kraft liner, and then covered by an outer protective coating of the varnish alone. Preferably, the dissipative ink is blue colored, which color is generally accepted in the industry as indicative of electrostatically dissipative material.
The present invention also contemplates a method for making commercial quantities of recyclable protective sheeting for use in packaging electrostatically sensitive devices, which sheeting has a high degree of uniformity of its static dissipative properties. The novel method comprises the steps of providing a carbon impregnated corrugated medium in continuous roll form, the corrugated medium having an electrical resistivity lesser than 10.sup.4 Ohms per square, characteristics. It is also desirable to provide such a static-safe fiberboard which is resistant to changes in relative humidity so as to maintain more uniform static dissipating properties in different environment. Additionally, it is highly desirable that the protective packaging should be recyclable by processes ordinarily used for the recycling of conventional non-conductive cardboards and papers.